It is known that semiconductor devices are employed as switches and they operate in “on” and “off” states. In the on-state the device can conduct high currents at modest voltages and the conduction losses are desired to be reduced. In the offstate the device can withstand the system maximum voltage with little or no current passing. There are generally two types of devices in the market, unipolar (MOSFET) where conduction is by majority carriers only, and bipolar where conduction is by majority and minority carriers. One fundamental problem is that to withstand a certain voltage a certain thickness of material is required (a drift region), set by a critical field for breakdown. Hence the higher the voltage the thicker and more resistive the drift region becomes. This sets the fundamental limit of a unipolar device in a given material because there is a compromise between voltage capability and device conductivity.
For these reasons wide band gap semiconductors with their much higher critical fields can be preferred to the standard silicon (Si). Of course, since Si is such an established engineering material, ways have been found to overcome this fundamental limit to some degree in the development of Superjunction MOSFETs. However the price paid as ever is much increased device complexity and higher manufacturing costs. The other way of overcoming this limit is to move from unipolar to bipolar devices. In this case the drift region resistivity is reduced in the on-state by the injection of minority carriers. The best known of these devices in the Si world is the Insulated Gate Bipolar Transistor (IGBT). In reality however this device is generally similar to a standard (non-SJ) MOSFET with the n+ Drain contact replaced by a p+ region which acts as an injector in the on-state. Of course there is a price to be paid for this enhanced performance in that the switching speed of an IGBT is much lower than that of a MOSFET because of the lengthy time required to remove the minority carriers from the drift region when switching the device off. This also increases switching losses.
Turning back to the trade-off between the breakdown voltage and specific on-resistance in unipolar device, consideration of basic materials properties and the structures of unipolar (MOSFET) devices are discussed below. FIG. 1 illustrates a recently published (N. Kaminski & O. Hilt, iPower2, Nov. 28, 2012, 18) graph illustrating trade-off between the specific on-resistance versus breakdown voltage for various Si and 4H-SiC devices.
Considering first the Si data, the mature Si-MOSFET technology shows clearly the difference between practical products and the theoretical limit. For example at 1000V the theory suggests 200 mΩ-cm2 while in practise 600 mΩ-cm2 is achieved. This data also shows the dramatic advantage of Super Junction technology with ˜75 mΩ-cm2 at 1000V.
The Si-IGBT and Si-IGCT data points relate to bipolar devices and demonstrate the advantages of this type of device in the breakdown voltage versus on-resistance compromise. Turning now to SiC another effect comes into play in that, unlike in Si, the resistance of the channel region under the MOSFET Gate becomes significant at “lower” voltages, as shown in FIG. 1.
It has been reported by Cree Inc. that the on-resistance in 4H-SiC devices is roughly half from the Channel region and half from the drift region. This would suggest that the current practical 4H-SiC unipolar limit for MOSFETs is five or six times higher than the theoretical value, probably reflecting the less than perfect material quality.
The SiC JFET data above suggests better performance than the MOSFET. However, this has to be regarded with a degree of caution since by their nature it is difficult to avoid a degree of bipolar action in a JFET. This of course may not be disadvantageous, except that this will bring into play other compromises.
The other bipolar SiC data points for Bipolar Transistors and IGBTs show some improvement over the unipolar limit, but nowhere near the several orders of magnitude difference between Si unipolars and Si bipolars.
Based on FIG. 1 and considering only device performance as governed by drift region consideration (and not costs) then it is clear that at 650V a simple structure MOSFET in SiC should outperform a Superjunction (SJ) Si MOSFET because the higher critical field of SiC permits a very thin drift region to be used. Further, since the SiC device cannot utilise SJ technology then the way is open to produce MOSFETs at higher voltages, as has been demonstrated by Cree Inc. All these unipolar devices conduct using electrons because of their high mobility and hence are termed “n-channel”.
However, at some point these unipolar devices will hit the drift region limit, although they are limited by other factors in 4H-SiC currently. Therefore it would be useful if the same “trick” could be employed in SiC to convert MOSFETs into IGBTs by adding a p+ injector. However, in fact this is (almost) impossible in 4H-SiC since all-pervasive nitrogen is an n-type dopant in SiC and the standard method of growing 4H-SiC crystals results in an n+ boule. Some work has been performed to produce p-channel IGBTs in 4H-SiC, forced by the presence of the n+ substrate.
Furthermore, the high channel resistance of 4H-SiC MOSFETs is a well-known problem, the best channel electron mobilities achieved are roughly 5% of the theoretical bulk value. A published review (Advances in SiC power MOSFET technology, Sima Dimitrijev, Philippe Jamet. Microelectronics Reliability 43 (2003) 225-233) summarises the idea that the high density of interface traps close to bottom of the conduction band causes the low mobility in 4H-SiC MOSFETs. The density of these interface traps is assumed to increase sharply toward the edge of the conduction band, where it reaches extremely high values as shown in FIG. 2.
It is an object of the present invention to address the problems discussed above.